[GHC] #11634: Bang patterns bind... unexpectedly. Deserves loud warning

[GHC]

#11634: Bang patterns bind... unexpectedly. Deserves loud warning
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           Reporter:  bgamari        |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.2.1
          Component:  Documentation  |           Version:  7.10.3
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 Consider the following,

 {{{#!hs
 {-# LANGUAGE BangPatterns #-}
 hello :: [Int] -> ()
 hello (!x:xs) = ...
 }}}

 To me it seems like the bang pattern should be applied to `x`; but it's
 not! In fact, this program is equivalent to,
 {{{#!hs
 hello (!(x:xs)) = ...
 }}}

 This strikes me as quite surprising, yet there is no mention of how `!` is
 supposed to bind in the [[http://downloads.haskell.org/~ghc/master/users-
 guide//glasgow_exts.html#ghc-flag--XBangPatterns|users guide]] nor the
 [[https://prime.haskell.org/wiki/BangPatterns|Haskell Prime proposal]].

 This behavior is slightly less surprising if you consider that the bang
 pattern `!Just x` is equivalent to `!(Just x)` (although this also strikes
 me as a bit unfortunate, although it is likely too late to do much about
 this).

 In sum: We need to document the current syntactic binding properties of
 `!` in the users guide.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11634>
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